The invention relates to a hydraulic valve play compensation element for the control drive of an internal combustion engine, which is provided as a reverse-spring/free-ball element, with a cylindrical housing, a cylindrical piston guided therein with sealing play, and a control valve, which is arranged between a low-pressure chamber of the piston and a high-pressure chamber of the housing on a piston head and which has a closing body, which can contact a valve seat surrounding an axial bore hole of the piston head when carrying out a stroke in the closing direction against the action of a control-valve spring and which can contact a contact surface of a valve cap surrounding the closing body in the opening direction.
Hydraulic valve play compensation elements are used for compensating for the play formed due to wear and tear or heat expansion when the cam stroke is transferred between transmission elements from a camshaft to a gas-exchange valve of the internal combustion engine. Through the use of the compensation element, a low-noise and low-wear operation of the valve drive and the best possible matching of the cam lift with the stroke of the gas-exchange valve should be achieved.
Such compensation elements each have a control valve, which is provided as a non-return valve and which has a closing body, for example a ball, and a control-valve spring applying a force onto the closing body. In the standard construction of the control valve, the control-valve spring applies a force on the closing body in the closing direction. Therefore, the control valve is predominantly closed and the return stroke of the valve lay compensation element is eliminated. In this configuration, there is the risk of pumping the compensation element upwards and producing a “negative valve lash.”
These disadvantages are avoided with control valves, whose control-valve spring applies a force on the closing body in the opening direction, or control valves, in which a spring is completely eliminated. Compensation elements with such a control valve are designated as reverse-spring elements due to the inverted arrangement of the control-valve spring or as free-ball elements due to the lack of a spring. These exert a positive influence on the thermodynamics, the pollutant emissions, and the mechanical stress on the internal combustion engine and are therefore being used increasingly.
In the standard construction, the control valve is predominantly closed in the base-circle region of the cam due to the spring force of the control-valve spring. In a reverse-spring element, however, the control valve in this region is held open by the force of the control-valve spring. In a free-ball element, the closing is not forced. Because such an element can be closed only by hydrodynamic and hydrostatic forces from the lubricating-oil flow set at the beginning of the cam lift and flowing from the high-pressure chamber to the low-pressure chamber, the element always has a return stroke before the beginning of the valve stroke of the gas-exchange valve. The size of the return stroke depends on the length of the closing time of the control valve at each engine rpm and this depends, in turn, on the viscosity/density of the lubricating oil, which here is used in a known way as hydraulic medium.
To close the control valve of a reverse-spring/free-ball element, a so-called critical lubricating-oil speed is necessary. This depends on the lubricating-oil viscosity and thus on the lubricating-oil temperature. For a high lubricating-oil viscosity/density, i.e., for a low lubricating-oil temperature, the critical lubricating-oil speed is lower and is therefore reached more quickly than for a lower lubricating-oil viscosity, thus a high lubricating-oil temperature. For a cold start, this leads to a shorter closing time of the control valve and thus to a smaller return stroke than for a warm-running engine. However, a small return stroke means a large valve overlap. This results in a high internal exhaust-gas recirculation, which causes noisy, low idle running. This can be improved by raising the idling rpm, but leads to costs in terms of pollutant emissions and fuel consumption.
Reverse-spring/free-ball elements of the type named above are known, for example, from EP 1 298 287 A2, JP 61-185607, and U.S. Pat. No. 4,054,109. These publications present compensation elements, for which the control valve has a ball bearing as a closing body. In these known publications, the closing body is guided in bore holes. However, because lubricating oil must flow around the closing body for closing the control valve, the guide gap cannot be selected arbitrarily small. Therefore, the closing body is not guided ideally, which leads to deviations in the closing behavior. On the other side, surrounding the closing body with oil is necessary to be able to define the closing behavior. Here, the closing behavior exhibits considerable thermal dependence.
Thus, for reverse-spring/free-ball elements, the closing body of the control valve is open in the base circle of the cam. For closing the control valve, a volume flow must flow past the closing body, which causes a pressure difference on the closing body, whereby this closes the control valve. To keep the tolerances of the generated return stroke small, the stroke of the closing body should fluctuate as little as possible. In addition, an eccentric gap between the closing body and the guide wall surrounding it has a negative effect on the flow, whereby the closing behavior is also influenced.